1. Field of the Invention
The present invention relates to a crystalline ceric oxide sol that is approximately monodisperse, and a process for producing the sol.
2. Description of the Related Art
Japanese Patent Laid-open No. Hei 10-95614 discloses a process for producing ceric oxide particles in which the process comprises reacting a cerium (III) salt with an alkaline substance in (OHxe2x88x92)/(Ce3+) molar ratio ranging from 3 to 30 to form a suspension of cerium (III) hydroxide, and immediately blowing oxygen or a gas containing oxygen into the suspension under atmospheric pressure at a temperature of 10 to 95xc2x0 C.
A sol comprising ceric oxide particles in a aqueous liquid prepared by the process disclosed in the Japanese patent publication has a specific surface area of 30 m2/g by gas adsorption method (BET method), and therefore the particle size thereof converted by BET method is 28 nm. The particle size of the sol measured by dynamic light scattering method is 306 nm. Accordingly, the ratio of the particle size measured by dynamic light scattering method to the particle size converted from a specific surface area by gas adsorption method is 11. Therefore, the sol is open to investigation on the sedimentation thereof, and there is a problem that the particles therein sediment and deposit on standing for a long time. Thus, in a case where the sol is used as an abrasive, it is necessary to re-disperse the sol by vigorously stirring it with a diaper and the like.
Accordingly, it is an object of the present invention to provide a process for producing a sol with high mono-dispersion properties. When the sol is used for polishing, it can prevent very small surface defects on the polished surface from occurring.
According to a first aspect of the present invention, there is provided a sol comprising crystalline ceric oxide particles, in which the particles have particle size I ranging from 10 nm to 200 nm and a ratio of particle size II to the particle size I ranging from 2 to 6, wherein the particle size I is a particle size converted from a specific surface area by gas adsorption method, and the particle size II is a particle size measured by dynamic light scattering method.
According to a second aspect of the present invention, there is provided a process for producing a sol comprising crystalline ceric oxide particles according to the first aspect, in which the process comprises the following steps A and B:
Step A of reacting a cerium (III) salt with an alkaline substance in (OHxe2x88x92)/(Ce3+) molar ratio ranging from 3 to 30 in an aqueous medium under an inert gas atmosphere to give a suspension of cerium (III) hydroxide, and immediately blowing oxygen or a gas containing oxygen into the suspension at a temperature of 10 to 95xc2x0 C. under an atmospheric pressure to give a sol comprising crystalline ceric oxide particles, in which the particles have particle size I ranging from 10 nm to 200 nm and a ratio of particle size II to the particle size I ranging 10 or more, wherein the particle size I is a particle size converted from a specific surface area by gas adsorption method, and the particle size II is a particle size measured by dynamic light scattering method; and
Step B of wet-grinding the sol obtained in step A.
According to a third aspect of the present invention, in the process of the second aspect, the alkaline substance is a hydroxide of alkaline metal, an organic base or a mixture thereof.
According to a fourth aspect of the present invention, in the process of the second or third aspect, the gas containing oxygen is air or a mixed gas of oxygen and nitrogen.
According to a fifth aspect of the present invention, there is provided a process for producing a sol comprising crystalline ceric oxide particles according to the first aspect, wherein the process comprises the following steps a and b:
Step a of calcining cerium carbonate at a temperature of 300 to 1100xc2x0 C. to give crystalline cerium oxide particles; and
Step b of wet-grinding the particles obtained in step a.
According to a sixth aspect of the present invention, in the process of the second to fifth aspect, the wet-grinding is carried out with a wet-ball mill, a sand grinder, an attritor, a perl mill an ultrasonic homogenizer, a pressure homogenizer or an ultimaizer.
According to a seventh aspect of the present invention, there is provided an aqueous slurry comprising organic-inorganic complex particles, in which the crystalline ceric oxide particles according to the first aspect are adsorbed on negatively charged polymer particles (polymer particles having negative charge).
The sol of the present invention comprises ceric oxide particles high in crystallinity. The ceric oxide particles have a specific surface area ranging from 4 to 83 m2/g by gas adsorption method (BET method), and the particle size thereof converted from the specific surface area value is 10 to 200 nm The particle size converted from specific surface area by the BET method (hereinafter referred to also as xe2x80x9cparticle size Ixe2x80x9d) shows the mean of the particle size of each particle.
Further, the particle size by dynamic light scattering method (hereinafter referred to also as particle size II) is measured with the aid of N4 (manufactured by Coulter Electronics, Inc.), DLS-6000 (manufactured by Otsuka Electronics Co, Ltd.), and the values range from 20 to 800 nm. The particle size determined by dynamic light scattering method shows that of particles in a sol. Therefore, when the particles are aggregated or held together, the particle size thereof is measured in such a state.
The crystalline ceric oxide particles in the sol of the present invention have particle size I ranging from 10 to 200 nm and a ratio of particle size II to a particle size I (that is, the ratio=(particle size by dynamic light scattering method)/(particle size converted from specific surface area by gas adsorption method)) ranging from 2 to 6. The low ratio means that the difference between the particle size converted from a specific surface area by gas adsorption method and the particle size measured by dynamic light scattering method is smaller than that of the prior sol comprising crystalline ceric oxide particles, and the particles in the sol of the present invention are relatively small state that they are aggregated and thus the sol is approximately monodisperse.
The sol of the present invention can be produced through the following steps A and B.
The step A for producing the sol of the present invention comprises reacting a cerium (III) salt with an alkaline substance in (OHxe2x88x92)/(Ce3+) molar ratio ranging from 3 to 30 in an aqueous medium under an inert gas atmosphere to give a suspension of cerium (III) hydroxide, and immediately blowing oxygen or a gas containing oxygen into the suspension at a temperature of 10 to 95xc2x0 C. under an atmospheric pressure to give a sol comprising crystalline ceric oxide particles, in which the particles have particle size X ranging from 10 to 200 nm and a ratio of particle size II to the particle site I ranging 10 or more.
The cerium (III) salts include, for example cerium (III) nitrate, cerium (II) chloride, cerium (III) sulfate, cerium (III) carbonate, ammonium cerium (III) nitrate. The cerium (III) salts may be used alone or in a mixture thereof
The alkaline substances include, for example alkaline metal hydroxides, such as sodium hydroxide and potassium hydroxide; organic bases, such as ammonia, amines and quaternary ammonium hydroxide. In particular, ammonia, sodium hydroxide and potassium hydroxide are preferred. These may be used alone or in a mixture thereof.
The gases containing oxygen include, for example air, oxygen, a mixed gas of oxidizing gas, such as oxygen with an inert gas, such as nitrogen. Air is preferable from the view points of economical efficiency and handling.
The step B comprises wet-grinding the sol obtained in the step A.
In addition, the sol of the present invention can be produced through the following steps a and b.
The step a comprises calcining cerium carbonate at a temperature of 300 to 1100xc2x0 C. to give crystalline cerium oxide particles. The cerium carbonate may be commercial cerium carbonate having the average particle size ranging from several xcexcm to 100 xcexcm.
The calcinating at a temperature below 300xc2x0 C. causes insufficient oxidation, and thus is not preferable. On the other hand, the calcinating at a temperature above 1100xc2x0 C. makes the primary particle size of the resulting ceric oxide 300 nm or more, and also is not preferable.
The step b comprises wet-grinding the crystalline cerium oxide particles obtained in the step a to give a sol comprising crystalline ceric oxide particles.
The steps B and b for producing the sol of the present invention comprise wet-grinding the crystalline ceric oxide sol or the crystalline ceric oxide particles obtained in the step A or a, respectively. The wet-grinding may be carried out with an apparatus, such as a wet-ball mill, a sand grinder, an attritor, a per) mill, an ultrasonic homogenizer, a pressure homogenizer or an ultimaizer.
The step B after the step A comprises wet-grinding the sol obtained in the step A until the ratio of particle size II to particle size I ranges from 2 to 6 while the particle size I obtained in the step A is maintained.
That is, the step B is carried out in order to divide particles that are held together weakly at a point without making the primary particle size of the crystalline ceric oxide particles smaller. Accordingly, particle size I of the particles after the step B is almost equal to that after the step A.
On the other hand, the step b comprising wet-grinding after the step a results in separation of particles the primary particle size of which are held together at plains in the step a without rupturing the primary particle size. Therefore, new planes are produced by peeling planes held together in the step b. Thus, the particle size I of the particles after the step b becomes smaller than that after the step a
It is preferable to mix quaternary ammonium ion (NR4+, wherein R is an organic group) with the sol comprising crystalline ceric oxide particles according to the present invention in a molar ratio of NR4+/(CeO2) ranging from 0.001 to 1 to give alkaline sol having high stability after removing impurities with the aid of cleaning, such as ultrafiltration, filter press cleaning or the like.
The quaternary ammonium ion can be obtained by adding quaternary ammonium silicate, quaternary ammonium halide, quaternary ammonium hydroxide or a mixture thereof, in particular quaternary ammonium silicate and quaternary ammonium hydroxide are preferable
Further, a small amount of acid or base can be added. In a case where acid is used, it is preferable to add water-soluble acid in a molar ratio of [H+]/[CeO2] ranging from 0.001 to 1 to give an acid sol having high stability The acidic sol has pH ranging from 1 to 6, preferably 2 to 6. The water-soluble adds include for example inorganic acid, such as hydrogen chloride, nitric acid or the like, organic acid, such as formic add, acetic acid, oxalic acid, citric acid, lactic acid or the like, an acidic salt thereof or a mixture thereof. Also, an alkaline sol may be obtained by adding a water-soluble base in a molar ratio of [OHxe2x88x92]/[CeO2] ranging from 0.001 to 1. The alkaline sol has pH ranging from 8 to 13, preferably 9 to 13. The water-soluble base include for example ethanolamine, diethanolamine, triethanolamine, N,N-dimethyl-ethanolamine, N-methylethanolamine, monopropanolamine and ammonia, potassium hydroxide, etc. in addition to quaternary ammonium hydroxide and quaternary ammonium silicate.
The sol comprising crystalline ceric oxide particles according to the present invention can be improved in stability as a sol by adding the above-mentioned base or acid after the step B or b However, it can be carried out to wet-grind or wet-crash with a wet-ball mill, a sand grinder, an attritor (e.g., manufactured by Mitsui Mining Ltd.), a perl mill (e.g., manufactured by Ashizawa Ltd.,), an ultrasonic homogenizer (e.g., manufactured by Nissei Ltd.), a pressure homogenizer (e.g., manufactured by SMT Ltd.) or an ultimaizer (e.g., manufactured by Sugino Machine Ltd.), etc. In order to stably perform the step B or b, it is preferable to add the above-mentioned base or acid after the step A or a
The sol comprising crystalline cerium oxide particles according to the present invention can contain further water-soluble polymer, anionic surfactant, nonionic surfactant or cationic surfactant. Illustrative example of them are as follows: water-soluble polymer, such as polyvinyl alcohol, acrylic polymer and ammonium salt thereof, methacrylic polymer and ammonium salt thereof, etc; anionic surfactant, such as ammonium oleate, ammonium laurate, triethanolamine lauryl sulfate, ammonium polyoxyethylene lauryl ether sulfate, etc.; nonionic surfactant, such as polyoxyethylene lauryl ether, polyoxyethylene sorbitan monolaurate, polyoxyethylene glycol disterate, polyethylene glycol monostearate, etc. The surfactants may be added in an amount of 0.01 to 100 parts by weight on the basis of 100 parts by weight of crystalline ceric oxide particles.
Observations by scanning electron microscope (SEM) or transmission electron microscope (TEM) indicate that the sol comprising crystalline ceric oxide particles according to the present invention has a particle size ranging from 20 to 300 nm by primary particle. In addition, the particles are measured of diffraction pattern with an X-ray diffraction apparatus after drying them at 110xc2x0 C. As a result of it, it is revealed that the particles have high crystallinity of cubic crystal system and are ceric oxide particles having a diffraction pattern with main peaks at diffraction angles 2xcex8=28.6xc2x0, 47.5xc2x0 and 56.4xc2x0 and described in ASTM Card No. 34-394. The particles in the sol have the particle size (BET converted-particle size) ranging from 10 to 200 nm that is converted from the specific surface area of the particles by a gas adsorption method (BET method), and a ratio of a particle size measured by dynamic light scattering method to the particle size converted from a specific surface area by gas adsorption method ranging from 2 to 6, therefore the sol comprises crystalline ceric oxide particles that are approximately monodisperse. Accordingly, even when the sol of the present invention is left for a long time, a part of the particles precipitates but can be easily re-dispersed by stirring or shaking and return to the initial dispersion state. Therefore, the sol of the present invention is stable for more than one year when it is kept at ordinary temperature.
The crystalline ceric oxide sol according to the present invention can be used as abrasives, ultraviolet absorbing materials, catalyst materials, fuel cell materials and the like in the form of sol comprising ceric oxide prepared by re-dispersing the particles in aqueous medium, water-soluble organic solvent or mixed solvent of water with water-soluble organic solvent
The crystalline ceric oxide sol according to the present invention is composed of crystalline particles that are dispersed in approximately monodisperse state. Accordingly, the ceric oxide particles according to the present invention having positive charge can be adsorbed uniformly on polymer particles having negative charge in a aqueous medium. Thus, it is expected that the cerium oxide-polymer complex is useful for abrasive. As the polymer particles having negative charge, it is preferable to use polymers having an anion functional group, such as hydroxyl, carboxyl, sulfonyl or the like thereon. The particle size of the polymer must be larger than that converted from a specific surface area of the crystalline ceric oxide particles of the present invention by gas adsorption method so that the ceric oxide particles will cover uniformly the polymers, and is preferably set to 5 times to more than 10 times the particle size of the crystalline ceric oxide particles. The particle size of the polymer usually ranges from 1 to 100 xcexcm. The crystalline ceric oxide particles having positive charge cover the polymer having negative charge. The amount of the particles and polymers may be selected in such a manner that a ratio by weight of the crystalline ceric oxide particles to the polymer is 0.05 and 3.0 as lower and upper limits, respectively. In the meantime, the crystalline ceric oxide particles may be added over the upper limit. However, in that case, a mixture of organic-inorganic complex particles and crystalline ceric oxide particles is prepared.
The crystalline ceric oxide sol according to the present invention may contain rare earth elements, such as lanthanum, neodymium, praseodymium and the like that do not deteriorate the characteristics of the sol.
The conventional sol comprising crystalline ceric oxide particles tends to occur sedimentation of particles therein because the particles are weakly held together to form large particles. Once the sedimentation occurs, it is difficult to re-disperse and return completely to the original state. In addition, when the conventional sol is used for an abrasive, it is not said that the surface polished with the abrasive has sufficient surface profile. On the other hand, the present invention provides a sol that is approximately monodisperse by wet-grinding the sol comprising crystalline ceric oxide particles prepared in advance to impart shearing force and impact force to them thereby dividing particles held together. And the divided particles are not held together nor aggregated again. The wet grinding can be carried out with any apparatus that generates shearing force and impact force rather than stirring force at high speed such as a disper and so on. As such an apparatus, it is preferable to use a wet-ball mill, a sand grinder, an attritor, a perl mill, an ultrasonic homogenizer, a pressure homogenizer, an ultimaizer or the like. The present inventors find out that the particles held together can be divided by wet-grinding colloidal particles of crystalline ceric oxide prepared in advance to impart shearing force and impact force, and therefore, in order to achieve it, the wet-grinding is carried out in the step B or b in the present invention.
The present invention resolves the problem that particles are held together by imparting shearing force and impact force to them, and makes possible to produce a sol comprising crystalline ceric oxide particles approximately monodisperse that was difficult to be produced in the prior art. When the crystalline ceric oxide sol with monodispersion properties is used for an abrasive, the abrasive does not sediment nor deposit in the line from a part feeding it to a polishing apparatus Therefore, the present invention enables to provide abrasives with constant and high quality that exert constant polishing performance because the abrasives do not sediment nor deposit even when they are used for a long time. The abrasives obtained in the present invention impart good polished surface to objects to be polished while maintaining high removal rate.